1. Field of the Invention
The present invention relates to a satellite broadcast receiving device, and more particularly, to a satellite broadcast receiving device having a circuit therein, in which two or more local oscillation circuits are simultaneously operated, in a low noise down-converter (LNB) used in satellite communication.
2. Description of the Background Art
A coaxial cable is generally used to direct a signal from a receiving antenna for satellite broadcasting to an indoor BS tuner. However, the coaxial cable cannot directly guide the radio wave received by the antenna to the indoor BS tuner.
It is required to use a metal tube called a waveguide in order to guide the radio wave in satellite broadcasting, which has a very high frequency. The use of the waveguide requires a big hole opened on a wall to guide a signal from the antenna to an indoor satellite receiver and also generates a large amount of attenuation, which makes it impractical. Thus, generally, an LNB installed near the antenna is used to reduce the frequency of a received signal to a frequency at a degree such that it can be guided by the coaxial cable, and to transmit the signal to an indoor satellite receiver. The indoor satellite receiver has a scramble decoder therein, which descrainbles the signal, and an image is displayed on a display machine.
Under circumstances where both analog and digital broadcasting exist, a broadband LNB is required to receive the both types of signals. A local oscillation circuit is used for converting a signal received from a satellite into a signal in a band that can be received by a satellite receiver on the ground. However, the band for the signal received from the satellite is broader than an output band of one local oscillation circuit. Therefore, two local oscillation circuits having different oscillation frequencies are generally used for reception.
For example, for the band of 10.7 to 11.7 GHz of the signal received from the satellite, the first local oscillation circuit having an oscillation frequency of 9.75 GHz is used to cover a frequency of 950 to 1950 MHz output from the LNB. Moreover, for the band of 11.7 to 12.75 GHz of the received signal, the second local oscillation circuit having an oscillation frequency of 10.6 GHz is used to cover a frequency of 1100 to 2150 MHz output from the LNB.
In the LNB structure, conventionally, two printed circuit boards were used, each of which were provided with one local oscillation circuit, to avoid the local oscillation circuits interfering with each other.
FIG. 10 shows a sectional structure of a conventional satellite broadcast receiving device.
Referring to FIG. 10, a board 234 is attached to one side of a sheet metal 246 having a thickness of d1, and a board 236 is attached to the other side thereof. Local oscillation circuits 212 and 218 are mounted on boards 234 and 236, respectively. A frame 242 is attached to cover board 234. Whereas, board 236 is covered by a chassis 232.
Conventionally, sheet metal 246 separated board 234 from board 236. The thickness of sheet metal 246 was, for example, approximately 2 mm. However, this could not keep a sufficient distance between the two boards.
Thus, when the two local oscillation circuits are simultaneously operated, a strong spurious signal is generated and a harmonic wave appears within a receiving band. The harmonic wave affects a signal from the satellite so that the signal cannot be normally transmitted to an indoor satellite broadcast receiver or the like, resulting in possible distortion of a picture on a television screen or the like.
FIG. 11 illustrates an arrangement of a contact pin 262 on board 236 in FIG. 10.
Referring to FIG. 11, in the conventional satellite broadcast receiving device, contact pin 262 is provided on board 236 in the vicinity of local oscillation circuit 218 including a dielectric resonator 272. By contact pin 262, local oscillation circuit 218 mounted on board 236 in FIG. 10 is supplied with a power potential from a power-supply circuit (not shown) on board 234.
FIG. 12 illustrates the shape of the conventional contact pin 262.
Referring to FIG. 12, an insulator 264 made of resin is attached in a middle portion of the rod-like contact pin 262 to determine the position with respect to the board.
FIG. 13 is a section view of a region around which contact pin 262 connects boards 234 and 236. Boards 234 and 236 are separated by metal chassis 246.
Referring to FIG. 13, insulator 264 made of resin is attached to the contact pin when the pin is connected to boards 234 and 236 by soldering or the like, to prevent contact pin 262 from falling off the board. When contact pin 262 is attached to resin 264, the position varies at which the resin is attached with respect to the pin. This causes variations of length D1 of contact pin 262 protruding from the boards. Moreover, some margin is required for the length of the pin in order to cover such variations, resulting in further increase of length D1 of the pin protruding from the boards.
As length D1 of the pin protruding from the boards is increased, the circuit becomes more susceptible to the radio wave, and thus a spurious signal easily occurs due to a mutual effect of the two local oscillation circuits attached to boards 234 and 236 respectively.